As society becomes increasingly environmentally conscious, there has been a growing trend to recycle a wide variety of consumable products including newspapers and other wood pulp-based materials.
Deinked waste fiber from chemical pulp products has been usually bleached by chlorine-based compounds, specifically chlorine and sodium hypochlorite. However, the industry starts moving away from chlorine for environmental concerns and this trend has been perceived as well in the recycled paper industry. Sodium hypochlorite is also perceived to be a chlorine compounds producer in effluent streams and air emissions and, as a result, is also an environmental concern. As such, there is now a growing interest to use ozone and/or hydrogen peroxide in the bleaching of recycled pulps.
It is known, for example, from "Cellulose Chemistry and Technology," 23-307-319 (1989) to bleach pulp with ozone. However, ozone degrades cellulose which consequently generates an important decrease of the polymerization degree of said cellulose which in turn decreases mechanical strength of the pulp.
In the article entitled "Upgrading of Waste Paper with Hydrogen Peroxide," O. Helmling, Recycling Paper from Fiber to Finished Products, pg. 714-724, is disclosed the upgrading of pulp from waste paper with hydrogen peroxide added during repulping; otherwise, the pulp will yellow particularly under alkaline conditions. Alkaline repulping is usually considered as being necessary for dissolving the pulp fiber structure and for dislodging ink particles from fibers by the swelling process.
However, it is necessary to stabilize hydrogen peroxide when it is added to the pulp, because hydrogen peroxide is decomposed by heavy metal ions and/or consumed by other organic fine material found in waste paper and/or dilution water. In early deinking processes carried out with hydrogen peroxide, up to 5% sodium silicate was added to the pulp as a stabilizer. However, because of scaling-up problems and reduced efficiency of the retention aids for paper manufacture, a number of chelating agents were developed as substitutes for sodium silicate as a stabilizer for hydrogen peroxide, as disclosed by Indresh Mathur in "Chelant Optimization in Deinking Formulation," 1st Research Forum on Recycling, October 1991, pp. 1-7. In any event, regardless of the stabilizer employed, it is believed to be necessary by the person killed in the art to use some stabilizer for hydrogen peroxide particularly in light of the metal ion decomposition problem referred to previously.
Presently, and as disclosed in the article entitled "Alternative Methods for Bleaching Post Consumer Waste Papers" J. E. Angulo, August 1991, Tappi Journal, there is a need for environmental-friendly process to bleach recycled pulp.
It is also disclosed in EP 514,901 a method to remove color from a recycled pulp made of waste papers to contact said pulp with oxygen or an oxygen containing gas which oxygen reacts with the color-causing compounds present in said pulp thereby bleaching said recycled pulp to make a recycled paper product. This oxygen bleaching step already well-known from the literature, might be followed by subsequent steps comprising (Z) ozone, hydrogen peroxide (P), etc. Prior to this oxygen stage or after, the pH of the pulp might be adjusted either at low pH or at high pH, avoiding the 8-10 pH zone. Subsequent Z, P, H stages might be provided.
The results indicate a slight bleaching action of oxygen alone, which action is enhanced by further stages, already known for their bleaching actions. The brightness obtained (which is not indicated whether it is prior to or after reversion) is rarely above 80, usually with long sequences.
It is known from the article entitled "Bleaching of secondary fibre pulps" --B. Van Lierop and N. Liebergott--First Research Forum on Recycling, Toronto, October 29-31, 1991--p 175-182 to treat recycled pulps from various fibre sources with Ozone (Z), hypochlorite (H), sodium hydrosulfite (Y) or formamidine sulphinic acid (FAS) bleaching stages.
One of the conclusions of this article is that an ozone bleaching stage followed by a sodium hydrosulfite (Y) stage which is a reductive stage is better than an ozone sodium hypochlorite sequence which is better than an ozone peroxide sequence; Table VII of this article also suggest that in order to obtain the best brightness of a pulp comprising recycled pulp, the ozone-hydrogen peroxide sequence must be followed by a sodium hydrosulfite (Y) sequence, i.e. a reductive sequence.
Contrary to the general teaching of the prior art, it has been discovered that applying reductive agents (and particularly sodium hydrosulfite) prior to applying an ozone/hydrogen peroxide sequence to a pulp comprising recycled pulp, enhances the effect of the overall combination of bleaching steps, i.e. improves the brightness of said pulp comprising recycled pulp after reversion. It has also been discovered that for certain recycled pulps the ozone sequence should be carried out at a pH which is greater than 7, and preferably between 8 and 11. For some other recycled pulps the ozone sequence should be carried out at a pH which is lower than 7 and preferably between 2 and 3.
In addition to that, the brightness of the pulp after reversion remains improved, sometimes even without using protective additives, usually added to avoid hydrogen peroxide decomposition by metallic ions.
As explained in the TAPPI publication TIS 0606-21--issued 1988--entitled "Recommended pulp bleaching stage designation method", an ozone (Z)/hypochlorite (H) sequence comprises usually two separate stages, a Z stage and an H stage with a washing or a pressing step between those two stages (first page--right column). In this case this sequence is designated as a ZH sequence. When those two stages are not separated by such a washing or pressing step, the sequence is named Z/H. When ozone and hypochlorite stages are done simultaneously, the sequence is designated as (Z+H) (see FIG. 7 of the article). However, when ozone and hypochlorite are added sequentially with the pulp in between points of addition, but with no washing or pressing, and before a common retention tower, the sequence is designated as (ZH)--see FIG. 10.
It is an object of the present invention to provide a chlorine-free bleaching process comprising a reductive step such as a sodium hydrosulfite (Y) step before the ozone (Z) and hydrogen peroxide (P) steps.
It is another object of the invention to provide an ozone (Z) and hydrogen peroxide (P) bleaching process which provides less brightness reversion than the usual ozone and hydrogen peroxide process.
This and further objects will be more readily apparent when considering the following disclosure and appended claims.